Metallurgical enclosures such as molten metal ladles, removable furnace roofs, as exemplified by the roof of a typical electric arc furnace, and other metallurgical enclosures or containers, are characteristically built with a steel shell containing a refractory lining normally constructed from refractory bricks. They are characterized generally by the fact that their linings are exposed either directly by contact or via radiation to the heat of molten metal and are customarily preheated prior to such exposure.
One universal reason for such preheating is to reduce as much as possible the thermal shock that the refractory lining would experience if exposed when cold to the heat of the molten metal. In addition, there may be other reasons for resorting to preheating.
For example, when a steel making or finishing furnace is tapped, the molten steel is poured into a ladle which is carried to a casting location, the steel cooling during transport, possibly to a temperature too low for casting. Additional cooling in the ladle may be caused by additional processing of the steel in the ladle. Compensation for ladle heat losses can be made by a compensating temperature increase of the steel in the steel making or processing furnace, but this involves furnace temperatures which are higher than is desirable from the viewpoint of desirably long furnace lining life. Preheating of the steel ladle can reduce heat loss while the steel is in the ladle and thus reduce the otherwise required high furnace operating temperature.
Heretofore the most effective preheating means has been the burning of gas flames within the enclosure being preheated. This has well-known objections. For example, a refractory lining cannot be preheated much above 1200.degree. C., the burnt gases must be exhausted from the preheating enclosure, thus resulting in heat waste and exhaust and dust problems, and because the flames usually operate in a fluttering manner, there is an uneven supply of heat and noise problem involved. Electric resistance heating has been proposed, but that practice is necessarily limited to drying a newly installed refractory lining. This follows from the fact that electric resistance heaters cannot radiate a great amount of heat.
It follows that the metallurgical industry has for a long time been confronted by the problem of preheating refractory lined furnace parts which when put into service are exposed to extremely high temperatures such as by actual contact of molten metal such as steel, radiation from molten steel and, in the case of an electric arc furnace roof, radiation from not only the molten metal but the arc as well.